[tsan] Fix Darwin GCD support after separation of Processor and ThreadState

Recent TSan changes (r267678) which factor out parts of ThreadState into a Processor structure broke worker threads on OS X.  This fixes it by properly calling ProcCreate for GCD worker threads and by replacing some CHECKs with RAW_CHECK in early process initialization.  CHECK() in TSan calls the allocator, which requires a valid Processor.

llvm-svn: 267864